[PATCH v4] mm, hugetlb: Fix simple resv_huge_pages underflow on UFFDIO_COPY

[Mina Almasry]

The userfaultfd hugetlb tests detect a resv_huge_pages underflow. This
happens when hugetlb_mcopy_atomic_pte() is called with !is_continue on
an index for which we already have a page in the cache. When this
happens, we allocate a second page, double consuming the reservation,
and then fail to insert the page into the cache and return -EEXIST.

To fix this, we first if there exists a page in the cache which already
consumed the reservation, and return -EEXIST immediately if so.

There is still a rare condition where we fail to copy the page contents
AND race with a call for hugetlb_no_page() for this index and again we
will underflow resv_huge_pages. That is fixed in a more complicated
patch not targeted for -stable.

Test:
Hacked the code locally such that resv_huge_pages underflows produce
a warning, then:

./tools/testing/selftests/vm/userfaultfd hugetlb_shared 10
	2 /tmp/kokonut_test/huge/userfaultfd_test && echo test success
./tools/testing/selftests/vm/userfaultfd hugetlb 10
	2 /tmp/kokonut_test/huge/userfaultfd_test && echo test success

Both tests succeed and produce no warnings. After the
test runs number of free/resv hugepages is correct.

Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx

---
 mm/hugetlb.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ead5d12e0604..76e2a6efc165 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4925,10 +4925,20 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 		if (!page)
 			goto out;
 	} else if (!*pagep) {
-		ret = -ENOMEM;
+		/* If a page already exists, then it's UFFDIO_COPY for
+		 * a non-missing case. Return -EEXIST.
+		 */
+		if (vm_shared &&
+		    hugetlbfs_pagecache_present(h, dst_vma, dst_addr)) {
+			ret = -EEXIST;
+			goto out;
+		}
+
 		page = alloc_huge_page(dst_vma, dst_addr, 0);
-		if (IS_ERR(page))
+		if (IS_ERR(page)) {
+			ret = -ENOMEM;
 			goto out;
+		}

 		ret = copy_huge_page_from_user(page,
 						(const void __user *) src_addr,
--
2.32.0.rc0.204.g9fa02ecfa5-goog